1. Technical Field
This disclosure relates to a semiconductor memory device, and more particularly, to a semiconductor memory device including a dynamic memory cell composed of a transistor with a floating body, and data write and read methods thereof.
2. Description of Related Art
A typical dynamic memory cell includes one access transistor and one data storage capacitor. When the data storage capacitor is charged, data “1” is stored; while when no charge is charged in the data storage capacitor, data “0” is stored. However, since the charge in the data storage capacitor is lost after a predetermined time elapses, a refresh (restore) operation should be performed.
In addition, since the typical dynamic memory cell needs the capacitor, when a memory cell array is includes the typical dynamic memory cells, layout area must be used for capacitors, thus there is a limit to how much the layout area of a semiconductor memory device may be reduced.
For this reason, a transistor having a floating body has been proposed. In this transistor, the floating body, which stores majority carriers, may need to be refreshed because the stored majority carriers are lost after a predetermined time elapses. Thus, although a memory cell including a transistor with a floating body does not include a capacitor unlike a typical memory cell, the memory cell including the transistor with the floating body operates similarly to the capacitor so that it can be used as a dynamic memory cell.
In other words, the transistor having the floating body constitutes one memory cell. Thus, assuming that a semiconductor memory device with a particular capacity is fabricated using memory cells with a transistor having a floating body, the semiconductor memory device including memory cells with transistor having a floating body has a smaller layout area than that of a semiconductor memory device including a typical memory cell.
FIG. 1 illustrates a conventional semiconductor memory device including a memory cell with a floating body. The semiconductor memory device includes memory cell array blocks BLK1 and BLK2, bit line selectors 10-11 to 10-1m and 10-21 to 10-2m, a reference bit line selector 12-1, level limiters 14-1 to 14-m, 14-(m+1), sense amplifiers 16-1 to 16-m, a reference voltage generator 18, comparators COM1 to COMm, latches LA1 to LAm, write back gates WBG1 to WBGm, read column selection gates RG1 to RBm, write column selection gates WG1 to WGm, and a reference write column selection gate RWG.
Each of the memory cell array blocks BLK1 and BLK2 includes memory cells MC and reference memory cells RMC. During a write operation, when a predetermined voltage (e.g., 1.5 V) is applied to the corresponding word line and a voltage higher than the predetermined voltage (e.g., over 1.5 V) is applied to the corresponding bit line, a lot of electron-hole pairs are generated due to impact ionization near the drain of an NMOS transistor of the corresponding memory cell MC. Here, electrons are absorbed in the drain of the NMOS transistor and holes are stored in a floating body, so that data “1” is written. That is, when the data “1” is written, the NMOS transistor operates in a saturation region. On the other hand, when a predetermined voltage (e.g., 1.5 V) is applied to the corresponding word line and a voltage (e.g., −1.5 V) lower than the predetermined voltage is applied to the corresponding bit line, the floating body and the drain of the NMOS transistor are forward-biased, so that the holes stored in the floating body are mostly discharged to the drain. As a result, data “0” is written.
When the data “1” is stored, the threshold voltage of the NMOS transistor decreases, and when the data “0” is stored, the threshold voltage of the NMOS transistor increases. In addition, during a read operation, when a predetermined voltage (e.g., 1.5 V) is applied to the corresponding word line and a voltage for operating of the NMOS transistor in a linear region (e.g., 0.2 V) is applied to the corresponding bit line, a current difference occurs in the corresponding bit line. By sensing the current difference, the memory cell reads data “0” and data “1.” When the memory cell stores data “1,” a bit line current generated when the data “1” is read increases due to a low threshold voltage. By comparison, when the memory cell stores data “0,” a bit line current generated when the data “0” is read decreases due to a high threshold voltage.
Each of the bit line selectors 10-11 to 10-1m and 10-21 to 10-2m selects one of the k bit lines BL1 to BLk of each of sub memory cell array blocks SBLK11 to SBLK1m and SBLK21 to SBLK2m in response to each of bit line selection signals BS1 to BSk and couples the selected bit line with the corresponding one of sense bit lines SBL1 to SBLm. Each of the reference bit line selectors 12-1 and 12-2 connects reference bit lines RBL1 and RBL2 of each of reference memory cell array blocks RBLK1 and RBLK2 with a reference sense bit line RSBL in response to the corresponding one of reference bit line selection signals RBS1 and RBS2.
Each of the level limiters 14-1 to 14-m, and 14-(m+1) cuts off the supply of the corresponding one of currents Ic1 to Ic(m+1) to the corresponding one of the sense bit lines SBL1 to SBLm and the reference sense bit line RSBL when the corresponding one of the sense bit lines SBL1 to SBLm and the reference sense bit line RSBL is at a higher voltage level than a limited voltage VBLR. That is, when the level of the limited voltage VBLR is set to 0.2 V, a voltage for a read operation is applied to the bit lines BL1 to BLk and the reference bit lines RBL1 and RBL2 due to the level limiters 14-1 to 14-(m+1) so as to allow the flow of the corresponding one of the currents Ic1 to Ic(m+1). Here, the reason that the limited voltage VBLR is set to a low level of 0.2 V is that when the limited voltage VBLR is set to a higher level than 0.2 V, the NMOS transistor with the floating body is biased in a saturated state so that when data “0” is read, data “1” may be read incorrectly due to impact ionization. The reference voltage generator 18 generates a reference voltage VREF corresponding to the current Ic(m+1). Each of the sense amplifiers 16-1 to 16-m senses the corresponding one of the currents Ic1 to Icm and generates a voltage corresponding to the sensed current. The reference voltage VREF generated by the reference voltage generator 18 is between voltages corresponding to the data “0” and the data “1,” which are output from each of the sense amplifiers 16-1 to 16-m. 
The write and read operations of the semiconductor memory device shown in FIG. 1 will now be described.
First, the write operation of the reference memory cells RMC will be described.
When a voltage of 1.5 V is applied to the word line WL11 and the reference bit line selection signal RBS1 is activated, the reference bit line RBL1 is coupled to the reference sense bit line RSBL. When a reference write column selection signal RWCSL is activated, an NMOS transistor N7 is turned on, and thus data transmitted to a write data line WD is transmitted through the reference sense bit line RSBL to the reference bit line RBL1. In this case, when write data has a voltage of −1.5 V, data “0” is written in the reference memory cell RMC connected between the word line WL11 and the reference bit line RBL1. In the same manner, the data “0” is written in all the reference memory cells RMC connected between other word lines and the reference bit lines RBL1. In addition, data “1” is written in all the reference memory cells RMC connected between the word lines WL11 to WL1n and WL21 to WL2n and the reference bit lines RBL2. In this case, write data may have a voltage of 1.5 V.
In other words, data “0” is written in the reference memory cells RMC connected to the reference bit lines RBL1 of the respective reference memory cell array blocks RBLK1 and RBLK2, and data “1” is written in the reference memory cells RMC connected to the reference bit lines RBL2 thereof. Thus, the reference memory cells RMC to the reference bit lines RBL1 and RBL2 of the respective reference memory cell array blocks RBLK1 and RBLK2 are used to generate the reference voltage VREF during the read operation.
Next, the write operation of the memory cell MC will be described. When a voltage of 1.5 V is applied to the word line WL11 and the bit line selection signal BS1 is activated, the bit line BL1 is connected to the sense bit line SBL1. When a write column selection signal WCSL1 is activated, an NMOS transistor N6 is turned on. In this case, when a voltage of −1.5 V is applied to the write data line WD, the voltage is applied through the sense bit line SBL1 to the bit line BL1 so that data “0” is written in the memory cell MC connected between the word line WL11 and the bit line BL1. On the other hand, when a voltage of 1.5 V is applied to the write data line WD, data “1” is written. In the same manner, all the memory cells MC perform the write operation.
The read operation of the memory cells MC will now be described. When a voltage of 1.5 V is applied to the word line WL11 and the bit line selection signal BS1 is activated, the bit line BL1 is connected to the sense bit line SBL1 so that a signal is transmitted from the bit line BL1 to the sense bit line SBL1. In this case, the reference bit line selection signals RBS1 and RBS2 are activated at the same time, and thus the reference bit lines RBL1 and RBL2 are connected to the reference sense bit line RSBL, and a signal is transmitted from the reference bit lines RBL1 and RBL2 to the reference sense bit line RSBL.
The level limiter 14-1 prevents the supply of current from an output node of the level limiter 14-1 to the sense bit line SBL1 when the sense bit line SBL1 is at a higher voltage level than the limited voltage VBLR, so that the sense bit line SBL1 remains at a lower voltage level than the limited voltage VBLR. Also, the level limiter 14-1 generates the current Ic1 corresponding to data stored in the memory cell MC. The level limiter 14-(m+1) prevents the supply of current from an output node of the level limiter 14-(m+1) to the reference sense bit line RSBL when the reference sense bit line RSBL is at a higher voltage level than the limited voltage VBLR, so that the reference sense bit line RSBL remains at a lower voltage level than the limited voltage VBLR. Also, the level limiter 14-(m+1) generates the current Ic(m+1) corresponding data stored in the reference memory cell RMC.
The sense amplifier 16-1 senses the current Ic1 and generates a sensing voltage sn1. The reference voltage generator 18 senses the current Ic(m+1) and generates a reference voltage VREF. The comparator COM1 is enabled in response to the sense amplifier enable signal SEN, compares the sensing voltage sn1 generated by the sense amplifier 16-1 with the reference voltage VREF, and generates sensing data. That is, when the sensing voltage sn1 generated by the sense amplifier 16-1 is lower than the reference voltage VREF, the comparator COM1 outputs a high-level signal to the corresponding node “a.” Inversely, when the sensing voltage sn1 is higher than the reference voltage VREF, the comparator COM1 outputs a low-level signal to the corresponding node “a.” The latch LA1 latches the sensing data.
In addition, when a read column selection signal RCSL1 is activated, NMOS transistors N2 and N4 are turned on. In this case, when the node “a” is at a high level, an NMOS transistor N5 is turned on and transmits low-level data to an inverted read data line RDB. On the other hand, when a node “b” is at a high level, an NMOS transistor N3 is turned on and transmits the low-level data to a read data line RD. That is, the low-level data is transmitted to the read data line RD or the inverted read data line RDB during the read operation.
After the read operation, when the write back signal WB is activated, an NMOS transistor N1 is turned on, so that high-level data is transmitted from the sense bit line SBL1 to the bit line BL1. Thus, a refresh operation is performed on the memory cell MC that stores data “1” and is connected between the word line WL11 and the bit line BL1. In the same manner, all the memory cells MC perform the read operation.
Thus, in order to perform a read operation, a conventional semiconductor memory device including memory cells with a floating body, a complicated circuit configuration including level limiters, sense amplifiers, comparators, latches, and write back gates as shown in FIG. 1 is needed.
Also, for the conventional semiconductor memory device, a long period of time is needed to perform a refresh operation. This is because a circuit configuration connected between a pair of sense bit lines, which is used for a read operation (or a refresh operation), is shared by k pairs of bit lines. In other words, one word line needs to be activated k times so that all the memory cells can perform the refresh operation.